Magnetic refrigeration is being considered as an alternative technique to gas compressor technology for cooling and heating based on engineering and economic considerations that indicate that magnetic regenerator refrigerators, in principle, using currently known and available magnetic materials are more efficient than gas cycle refrigerators and thus can yield savings in the cost of operation and conservation of energy.
In magnetic regenerator refrigeration for the liquefaction of hydrogen gas, a demonstration magnetic regenerator is designed to operate over a temperature region from about 20 K to about 90 K with heat rejection typically into liquid nitrogen heat exchanger. An active magnetic refrigerant proposed for the demonstration regenerator for the low temperature stage (operating at approximately 20 K to 55 K) to achieve liquefaction comprises a GdPd alloy while a higher temperature stage (operating at approximately 45 K to 90 K) comprises GdNi.sub.2. The low temperature stage GdPd magnetic refrigerant is disadvantageous from a cost standpoint in that 50 atomic % of the alloy comprises expensive Pd metal. Moreover, although the GdPd alloy exhibits useful magnetic entropy, there are several other heavy rare earth lanthanides (e.g. Tb, Dy, Ho, and Er) that exhibit magnetic entropy values approximately 35% larger than that of Gd and thus theoretically offer improved properties for magnetic refrigeration, provided all of the magnetic entropy is associated with the ferromagnetic ordering process on which magnetic refrigeration is based. Magnetic materials including Tb, Dy, Ho, and Er appear to have been neglected as candidate magnetic refrigerant materials as a result of the belief that an appreciable fraction of the magnetic entropy of these materials is associated with crystalline electric field effects, not ferromagnetic ordering, and thus would be less than the magnetic entropy attributed to ferromagnetic ordering observed in Gd in which there are no crystal field effects. Moreover, materials containing Tb, Dy, Ho, and Er exhibit a disadvantageously lower ferromagnetic ordering temperature than the corresponding Gd materials.
U.S. Pat. No. 4,829,770 describes an attempt to provide a magnetic refrigerant material exhibiting magneto-thermal properties for a magnetic regenerator refrigerator based on the Ericsson thermodynamic cycle. In particular, the patent describes a complex magnetic refrigerant that must include at least three distinct magnetic aluminide compounds in powdered or sintered admixture or in a multi-layered arrangement. The magnetic compounds are selected from aluminides of Gd, Tb, Dy, Ho and Er. The magnetic refrigerant mixture or multi-layer described in the patent is formulated for use specifically in an Ericsson thermodynamic cycle refrigerator and it requires at least three layers of different refrigerant materials.
U.S. Pat. No. 5,435,137 describes a ternary magnetic refrigerant comprising (Dy.sub.1-x Er.sub.x)Al.sub.2 for a magnetic regenerator refrigerator using the Joule-Brayton thermodynamic cycle spanning a temperature range of about 60 K to about 10 K where the ternary alloy can be used as a sole refrigerant in that range as a result of its magnetocaloric properties.
An object of the present invention is to provide a dual stage magnetic regenerator for a magnetic refrigerator and refrigeration method using the Joule-Brayton thermodynamic cycle and using a combination of active magnetic refrigerant materials that provide significantly improved regenerator efficiency.